Genetics and Miniature horsesMunro Marx

Unistel Medical laboratories

Miniature Horses

• Miniature horses are bred all over the world

• They are particularly popular in Europe and

the USA

• Over the last number of years they have also

become very popular in South Africa

• Miniature horses qualify by height

• Normally between 34 to 38 inches (86 cm

tot 97 cm)

• Normally measured from the shoulder

to ground

• Miniature horses retain all their true

horse attributes

Miniature Horses

• The oldest living miniature horse on record Angel

who lived to be over 50.

• The world's smallest horse is also a horse affected

by dwarfism

• Thumbelina stands 17 inches (43 cm) tall.

• Miniature horses have endured selective inbreeding

over the past 350 years .

• It is estimated that there are more than 100,000

Miniature Horses in the United States.

Thumbelina

Angel

Dwarfism and miniatures• Dwarfism is a concern within the miniature horse world.

• Dwarfism is defined:

• “underdevelopment of the body characterized by an abnormally short stature often with underdeveloped limbs and other defects”

• Dwarf horses (just a few issues)

• generally have incorrect conformation

• may have significant health and soundness issues.

• can have trouble walking,

• can have severe dental problems that can lead to episodes of colic.

• can suffer from chronic pain caused by their malformed limbs and spine

• are susceptible to obesity and can have difficulties carrying a foal

Dwarfism and miniatures

• Many miniature horse registries try to avoid

accepting miniatures affected by dwarfism,

especially for breeding stock.

• Dwarfism is a recessive genetic disorder

• Both parents therefore must pass on an

affected allele to the offspring.

• Equine dwarfism is prevalent in the miniature

horse breed (also present in normal horse

breeds)

• Recorded in Friesians, mustangs, Shetland

ponies and even the miniature donkey.

Genetics of Dwarfism in Miniatures

• There are for types dwarfism in miniature horses

of which Chonroplasia is most prevalent.

• Chondroplasia is a recessively inherited genetic

disorder

• Four different causative mutations in the ACAN

gene have been characterised namely D1, D2, D3

and D4

• The D1 mutation has been found to be lethal in

combination with any other mutation.

• It is therefore advisable not to breed carriers of

the D1 mutation to any other ACAN gene

mutation carriers.

Dwarfism and miniatures

• In the miniature horse breed, dwarfism is estimated to be in over 50% of the population and

affects all miniature horse bloodlines.

• Dwarfism cannot be culled by phenotypic characteristics because of the fact that the carriers

have normal phenotypes and have even lead successful show careers.

• D1D1 Dwarfs. Abort early in pregnancy

• D1/D2; D1/D3 and D1/D4 foals aborted during pregnancy

Dwarfism and miniatures

• D2D2 Dwarfs, can live normal lives with some problems

• D2/D3; D2/D4 (live) and D2/D1 (aborted during pregnancy)

Dwarfism and miniatures

• D3D2; D3D4 and D3D1 (aborted during pregnancy)

• D4/D2; D3/D4 and D4/D1 (aborted during pregnancy)

Miniature horse parentage verification

• Breeders should be encouraged to provide proof of parentage

of all foals.

• Ensures the integrity of not only the herd book of the

breeder but also the society data base

• Ensures the integrity of the seller and clients will be

prepared to pay a premium for proof of a bloodline.

• DNA is the technology of choice

• A DNA profile is the only tamper proof form identification

• Provides proof of identity in cases of theft

• Important to include both parents in the test, especially in

closely related matings.

Miniature horse parentage verification

• Horse DNA profiles compiled using 12 STR markers

• Parentage only verified if 12 from 12 markers are

matched.

Sex marker to confirm the sex of the horses

tested. Foal is female with no H marker for the

Y chromosome

In Summary

• Dwarfism is estimated to be present in over 50% of the

population and affects all miniature horse bloodlines.

• ACAN gene mutations are the most common cause of dwarfism

in the miniature horses

• This is an unwanted recessive genetic trait. Many miniature

horse registries try to avoid accepting miniatures affected by

dwarfism, especially for breeding stock.

• Breeders should be encouraged to test their horses and make

use of the information to make wise breeding (mating) decisions

• Breeders must make the results of tests known to clients

(Consumer Protection Act)

• Progeny of parents tested clear do not have to be tested again.

• Breeding should always be verified.

Thank you for listening.

Any questions?